Rollup merge of #97498 - ijchen:master, r=Mark-Simulacrum

Corrected EBNF grammar for from_str

Hello! This is my first time contributing to an open-source project. I'm excited to have the chance to contribute to the rust community 🥳

I noticed an issue with the documentation for `from_str` in `f32` and `f64`. It states that "All strings that adhere to the following [EBNF](https://www.w3.org/TR/REC-xml/#sec-notation) grammar when lowercased will result in an `Ok` being returned. I believe this is incorrect for the string `"."`, which is valid for the given EBNF grammar, but does not result in an `Ok` being returned ([playground](https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=09f891aa87963a56d3b0d715d8cbc2b4)). I have simplified the grammar in a way which fixes that, but is otherwise identical.

Previously, the `Number` part of the EBNF grammar had an option for `'.' Digit*`, which would include the string `"."`. This is not valid, and does not return an Ok as stated. The corrected version removes this, and still allows for the `'.' Digit+` case with the already existing `Digit* '.' Digit+` case.

library/core/src/num/dec2flt/mod.rs

@@ -126,7 +126,6 @@ macro_rules! from_str_float_impl {
             /// ```txt
             /// Float  ::= Sign? ( 'inf' | 'infinity' | 'nan' | Number )
             /// Number ::= ( Digit+ |
-            ///              '.' Digit* |
             ///              Digit+ '.' Digit* |
             ///              Digit* '.' Digit+ ) Exp?
             /// Exp    ::= 'e' Sign? Digit+